Flight control feature of Boeing 737 MAX under scrutiny after Lion Air accident

13 November 2018

The U.S. Allied Pilots Association (APA), which represents American Airlines Group Inc. pilots, alerted its members of a new flight control feature of the Boeing 737 MAX models. This feature, called MCAS (Maneuvering Characteristics Augmentation System), applies nose down stabilizer in specific conditions when the aircraft nears a stall. The APA quoted a Boeing message as follows:

MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.

A line of inquiry into the October 29 accident involving a Lion Air Boeing 737 MAX 8 focusses on a possible issue with one of the aircraft’s AOA (Angle of Attack) sensors. A subsequent Emergency Airworthiness Directive issued by the FAA stated that erroneously high single angle of attack (AOA) sensor input could result in “repeated nose-down trim commands of the horizontal stabilizer”. Which would be the MCAS commanding these nose-down trim commands.

APA said the logic behind MCAS was not mentioned in training or in any other manuals or materials. Safety Committee Chairman Capt. Michaelis stated: “It’s pretty asinine for them [Boeing] to put a system on an airplane and not tell the pilots who are operating the airplane, especially when it deals with flight controls.”

Operational Evaluation Report

The MCAS system was briefly mentioned in the Operational Evaluation Report, prepared by the Brazilian aviation authorities, ANAC. These reports are prepared by aviation authorities, among others, to determine pilot qualification and type rating requirements including training, checking, and currency requirements.

The Brazilian ANAC report, dated January 10, 2018 contains Operator Difference Requirements (ODR) Tables, listing design differences between the Boeing 737-800 and the Boeing 737-8 (MAX), as proposed by The Boeing Company and validated by ANAC.

One of these pertained to MCAS:

Level B Training is defined by the FAA as ‘Level B Training. Level B training is applicable to related aircraft with system or procedure differences that can adequately be addressed through aided instruction. At level B,aided instruction is appropriate to ensure pilot understanding, emphasize issues, provide astandardized method of presenting material, or aid retention of material following training.Level B aided instruction can utilize slide/tape presentations, computer based tutorial instruction,stand-up lectures, or video tapes.’

The Operational Evaluation Report prepared by Transport Canada, dated 30 November 2017, contained the same table, without the line referring to MCAS.